Uranium monocarbide-plutonium mononitride nuclear fuel



United States Patent Moulineaux, France, assignors to Connnissariat a 5 lEnergie Atomique, Paris, France No Drawing. Filed Oct. 27, 1964, Ser. No. 406,899 Claims priority, application France, Nov. 5, 1963, 9 7

6 Claims. Ci. 252-3014) The present invention has for its object a method of preparation of fuel material which is intended for use especially in a breeder reactor which employs a uraniumplutonium fuel cycle.

This fuel material consists of a compound sintered product which corresponds to the general formula UC-PuN, the relative proportions of UC and of PuN being between 0 and 100% by Weight. In fact, only the use of fuel material of this kind determines the relative proportions of uranium carbide and plutonium nitride. For example, in the fabrication of breeder reactor fuel elements, the weight percentage of plutonium nitride remains advantageously Within the range of 2 to 25%.

The UC-PuN mixture has substantial advantages over other mixed fuel compositions of uranium and plutonium.

In fact, fuel elements of this type have a very high degree of purity owing to the existence of a single phase. In addition, they have good storage behavior and very good compatibility with stainless steel, thereby permitting the use of this material for the purpose of cladding said fuel elements. Moreover, their face-centered cubic structure which is statically isotropic, endows them with very good resistance to thermal cycling. Finally, the said fuel elements are stable up to at least 1800 C. without loss of plutonium as a result of volatilization.

It is already known to prepare uranium monocarbide and plutonium mononitride, for example by means of the methods described in French Patents No. 1,303,488, of June 19, 1961, and No. 1,335,556, of July 4, 1962. It could therefore have been considered possible to fabricate fuel elements according to the invention by mixing uranium carbide and plutonium nitride powders in suitable proportions and by sintering the mixture. However, experience has revealed that the diffusion which takes place is very poor, that a single phase is not achieved and, worse still, that it proves impossible to prevent the formation of inclusions of higher carbides of the type (U, Pu) C and (U,Pu)C

In order to overcome the disadvantages noted above,

the invention is directed to a method of preparation of fuel material consisting of a mixture of plutonium mononitride and uranium carbide, characterized in that plutonium mononitride is mixed with carbon and uranium hydried in suitable respective proportions for the purpose of fabricating uranium monocarbide and in that the mixture of plutonium mononitride and uranium monocarbide which is obtained is sintered under suitable conditions so as to obtain a density in the vicinity of theoretical density.

In accordance with a first preferred mode of ope-ration of the method, the above reaction is performed in a single operation in said reaction mixture which is shaped after incorporation of 1% naphthalene and 0.5% nickel, at a temperature within the range of 1200 to 1700" C. The first mode of operation makes it possible to obtain fuel elements having a density which remains Within the range of 11.8 to 12.2.

According to a second preferred mode of operation of the method, fuel materials are obtained which have a Patented Mar. 14, 1967 density within the range of 12.4 to '13 as a result of the following sequence of operations:

The mixing of reagents is carried out by grinding of the constituents; v

The powder which is obtained is compacted into pellets without addition of binder, under a pressure within the range of 2,000 to 10,000 bars;

The said pellets are then homogenized in vacuo at a temperature within the range of 1300 to 1500 C. over a period of time which varies from 2 to 6 hours. This homogenization process is in fact a diffusion process which, under these conditions, is total;

The said homogenized pellets are then ground, and the powder obtained has added to it 1% naphthalene (which serves as binder) and 0.5 nickel (which serves as sintering adjuvant);

The said mixture is then compacted into pellets under a pressure of 8,000 to 15,000 bars;

The said pellets which are thus produced to final dimensions are then sintered at a temperature within the range of 1200 to 15 00 C. under a pressure within the range of 10 to 10 millimeters of mercury over a period of time varying between 2 and 6 hours. In order to increase the density of the pellets which have thus been sintered, it is possible if so desired to subject them to an additional annealing treatment at a temperature of the order of 1600 C. for a period of time which can vary between 2 and 6 hours.

There will now be given a number of diiferent examples of the method according to the invention, these examples being given Without intended limitation. The practical arrangements which will be described in connection with these examples must be considered as forming part of the invention, it being understood that any equivalent arrangements could equally well be employed without thereby departing from the scope of this invention. In these examples, the weights given are rounded off to the nearest milligram and the densities given only to the first decimal point.

Example I This example relates to a composition of fuel material containing 15% by weight of PuN.

The PuN employed has a ratio N/Pu=0.98.

There was mixed in the grinding mill 26.173 grams of PuN containing 143.121 grams of UH and 7.126 grams of C. The mixture delivered from the grinding mill was then pressed into pellets without addition of binder, under a pressure of 5,000 bars. The pellets were then homogenized in a vacuum of 10 mm. Hg at a temperature of 1400 C. for a period of four hours. The said pellets were transferred to the grinding mill after addition of 1% naphthalene and 0.5% Ni, and the powder thus produced was pressed into pellets under a pressure of 10,000 bars. These new pellets were then sintered for a period of four hours at a temperature of 1400 C. in a vacuum of 5 X l0 mm. Hg.

The density of these sintered products was equal to 12.4.

Example 11 Under the same conditions as in the previous example, a fuel material containing 15% PuN was fabricated from the following compounds:

Grams PuN (ratio N/Pu=0.995) 20.945 C 5.697 UH 114.415

the density of the sintered pellets obtained was 12.5.

3 Example III Under the same conditions as in Example I, a fuel material containing 12.37% PuN was fabricated from the following compounds:

The sintered pellets obtained were then subjected to an annealing treatment at 1600 C. for a period of 4 hours.

The density obtained was 12.9.

What we claim is:

1. Method of preparation of fuel material having a plutonium and uranium base for utilization in nuclear reactors, the steps of mixing plutonium mononitride with carbon and uranium hydride in suitable respective proportions, heating the mixture and producing a mixture of plutonium mononitride and uranium monocarbide and sintering the mixture of plutonium mononitride and uranium monocarbide to a density in the vicinity of theo retical density.

2. Method in accordance with claim 1, including the step of incorporating 1% naphthalene and 0.5% nickel in the mixture of plutonium mononitride and uranium monocarbide, said sintering being conducted at a temperature within the range of 1300 to 1700 C.

3. Method for preparation of fuel material, the steps of mixing ground plutonium mononitride, ground carbon and ground uranium hydride in suitable proportions to obtain the desired composition, compacting the powder thus obtained into pellets at a pressure within the range of 2,000 to 10,000 bars, homogenizing the pellets obtained by heating to a temperature Within the range of 1300 to 1500 C. for a period of time which varies between 2 and 6 hours, grinding said homogenized pellets after incorporation of 1% naphthalene and 0.5 nickel, compacting the powder thus obtained into pellets under a pressure within the range of 8,000 to 15,000 bars, and then sintering the pellets obtained at a temperature within the range of 1300 to 1500 C. under a pressure within the range of 10- to 10- mm. of mercury for a period of time which varies from 2 to 6 hours.

4. Method in accordance with claim 3, wherein the sintered pellets are subjected to an additional annealing treatment at a temperature of the order of 1600 C. for a period of time which varies between 2 and 6 hours.

5. Fuel material for breeder reactors consisting of a mixture of uranium monocarbide and plutonium mononitride.

6. Fuel material for breeder reactors consisting of a mixture of uranium monocarbide and plutonium mononitride, said mixture comprising by weight from 2 to 25% plutonium nitride.

References Cited by the Examiner UNITED STATES PATENTS 3,044,946 7/1962 Litton 252301.1 X 3,230,177 1/1966 Blum et al 252-30l.l 3,238,140 3/1966 Hedger et al 25230l.1 3,275,564 9/1966 Pascard et al. 252301.l

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

S. J. LECHERT, 111., Assistant Examiner. 

1. METHOD OF PREPARATION OF FUEL MATERIAL HAVING A PLUTONIUM AND URANIUM BASE FOR UTILIZATION IN NUCLEAR REACTORS, THE STEPS OF MIXING PLUTONIUM MONONITRIDE WITH CARBON AND URANIUM HYDRIDE IN SUITABLE RESPECTIVE PROPORTIONS, HEATING THE MIXTURE AND PRODUCING A MIXTURE OF PLUTONIUM MONONITRIDE AND URANIUM MONOCARBBIDE AND SINTERING THE MIXTURE OF PLUTONIUM MONONITRIDE AND URANIUM MONOCARBIDE TO A DENSITY IN THE VICINITY OF THEORETICAL DENSITY. 